Apple II+ restoration

6 posts / 0 new
Last post
ericF's picture
Offline
Last seen: 1 year 1 month ago
Joined: Sep 22 2013 - 16:57
Posts: 28
Apple II+ restoration

I'm diving into my first Apple II restoration job on a machine that is just plain dead. Power on the PSU hums, but no light and no bell (let alone video). I was hoping to get some initial directions and some information on this particular machine. It's a fairly late serial number A2S2 (dating to late 1982) with the rev D RFI motherboard. There are a number of interesting upgrades, including a "lowercase chip", a "Repeaterrr" board, and a few expansion cards (Microsoft RAM, Apple Super Serial, Applied Engineering VideoMaster.)

I've started with the power supply and determined the following:

1. The voltages on the motherboard are all off. With reference to ground on pin 16 of the last memory chip the following voltages are measured:
pin 1 -0.4V
pin 8 1.8V
pin 9 0.69V

So the relative magnitudes and polarities are correct (pin 1 should be -5V, pin 8 should be +12V, pin 9 should be +5V) but the voltages are obviously way off. I confirmed these voltages on the expansion slots as well.

Searching previous discussions here and elsewhere, it is mentioned that this probably indicates a short on the motherboard and not necessarily a problem with the PSU (http://www.vintage-computer.com/vcforum/archive/index.php/t-13620.html)

2. After this, I checked the resistances on the pins relative to ground:
Measured resitances:
Pin 1: 47.5 Ohm
Pin 8: 51.2 Ohm
Pin 9: 5 Ohm

The above discussion suggests that anything below 10 Ohm indicates a short. So can I conclude that the +5V line is shorted? Is this likely to be on the motherboard or in the PSU?

3. The power supply "hums" (sounds like 60Hz), but I'm not sure it is "chirping". It is somewhat "metallic" sounding. I'm not sure if this is the "clicking" some refer to that could indicate a problem in the low voltage side of the PSU circuit.

4. All internals look ok on both the motherboard and PSU. No obvious blown capacitors and the PSU fuse is good.

A few other things I've noted on this machine.

1. The PSU unit is unlike any I've seen. It is black, has no Apple labeling, and appears to be a different internal design than that described in the SAMS book or red book. The model is "PS-A" and it is made in Taiwan (see attached photo). I'm guessing that this is a replacement PSU? The back has a switch that I assume allows the power supply to be selected between 110 and 240V. There is a plate that is screwed down holding the switch in place and a label that says "110V". So perhaps this was a power supply that could be fitted in either the US or Europlus models?

2. The RAM installed in this machine is not made up of the 4116 chips, but National Semiconductor 5290N-3. The reference sheets I find on these indicate that these are 16K x 1 chips? This does not make sense since they should be 16K x 8.

3. For what its worth, documentation I have for this machine indicates that it was manufactured in Texas. Do the different parts correspond to different manufacturing sites or were all machines assembled in Texas at that point?

Any information would be greatly appreciated!

eric F

speedyG's picture
Offline
Last seen: 1 year 6 months ago
Joined: Nov 16 2011 - 07:45
Posts: 2494
Re: Apple II+ restoration

hello EricF,
first of all some points:
unless the powersupply does not deliver the correct voltages its better to start up with that device disconnected...

at the other side the powersupply can only be measured reliable with some kind of load....
in most such cases a adaptor is nowadays used by adding load to +12V, +5V and GND by
making an adaptor from the plug to the usual powercable used for old IDE harddisks....
adding at the other side lab-connectors...

then the first task will be to check the powerdevice... buy this also any shotsuts at the mainboard are not in the game... if the powersupply delivers reliable voltages ( including also the negativ coltages )then next step can be performmed by checking the mainboard...

that next step should be performmed still with the disconnected mainboard, by removing all chips from board to
antistatic foam besides of the mainboard and locating shortcuts, if there are ones ( for example in electrolytic
capacitors and coils close to the powerplug connector...

if any existing shortcuts on mainboard have been detected and removed - the chips should be reinserted in the very basic configuration with all logic chips and CPU but only first 16 kB RAM inserted in ROW C.

If any trouble is recognized ( when starting the computer - but not ending at the prompt with normal display - it will be the next step to identify bad chips or bad RAM.

When ending at the prompt the computer may be upgraded first with another row of additional RAM in Row D.

If that startup is also OK the next step will be to upgrade to 48 kB by adding the RAM in Row E.

Then you may start testing a boot with DOS-diskette and 48 kB RAM and testing system with dealers testing disk.

If that completes next step will be to add the languagecard and then testing again the languagecard with testdisk.

Next step will be to test booting of ProDOS disk.

Thereafter the other interface cards may be added step by step each time again testing the added card before inserting next card...

By limiting tested parts in different steps you protect other parts from being damaged by damaged parts...
and you limit to possible mistakes

sincerely speedyG

dorkbert's picture
Offline
Last seen: 4 hours 14 min ago
Joined: Apr 12 2009 - 16:33
Posts: 325
Re: Apple II+ restoration

[double post - delete]

dorkbert's picture
Offline
Last seen: 4 hours 14 min ago
Joined: Apr 12 2009 - 16:33
Posts: 325
Re: Apple II+ restoration

If you want to muck with the power supply for a learning experience, go for it. Otherwise replacements are available (on and off) for $15 on fleaBay. In the long run, it's probably better to go with an Apple power supply if only because documentation is widely available.

ericF's picture
Offline
Last seen: 1 year 1 month ago
Joined: Sep 22 2013 - 16:57
Posts: 28
Re: Apple II+ restoration

Thanks, SpeedyG! That gives me some confidence to move ahead.

If anyone has any thoughts on the origin of the power supply, I'd love to hear about it.

speedyG's picture
Offline
Last seen: 1 year 6 months ago
Joined: Nov 16 2011 - 07:45
Posts: 2494
Re: Apple II+ restoration

well o.K. that kind of powersupplies have often been used in clones from taiwan...
the advantage was that they often had more power... :
instead of the 1,5 Ampere at the +12 Volt they often delivered 2 Ampere ( this one even 3 Ampere ),
and instead of the 4 Ampere at the +5 Volt they often had output of 5 to 7,5 Ampere ( this one even 6 Ampere )
and instead of the normal 0,4 Ampere at -5V and -12V - this one even has output of 1 Ampere each ....

because the original powersupply was rather weaker the user changed that supplies in former days against the stronger ones, if they had loaded their Apple with more cards or added more drives to them .... in such cases often the original
powersupply got very hot or even surrendured....

but the problem with this supplies - in most cases there is no documentation and no circuitplan availiable and therefor
in most cases such supplies can be repaired only by techicians with advanced knowledge to switching power supplies....
therefor i would also recommend to search for a powersupply known to be in working condition...

sincerely speedyG

Log in or register to post comments